The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for non-contrast enhanced magnetic resonance angiography (“MRA”).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. Usually the nuclear spins are comprised of hydrogen atoms, but other NMR active nuclei are occasionally used. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1; also referred to as the radiofrequency (RF) field) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped” into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation field B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomenon is exploited.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged experiences a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The emitted MR signals are detected using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Magnetic resonance angiography (MRA) and, related imaging techniques, such as perfusion imaging, use the NMR phenomenon to produce images of the human vasculature or physiological performance related to the human vasculature. There are three main categories of techniques for achieving the desired contrast for the purpose of MR angiography. The first general category is typically referred to as contrast enhanced (CE) MRA. The second general category is phase contrast (PC) MRA. The third general category is time-of-flight (TOF) or tagging-based MRA.
Contrast-enhanced MRA techniques require venous cannulation and the use of exogenous contrast material. Such agents are costly and expose the patient to added safety risks, namely, nephrogenic systemic fibrosis. Non-enhanced techniques for MRA are helpful for the evaluation of suspected vascular disease in patients with impaired renal function, since they avoid the risk of nephrogenic systemic fibrosis.
Examples of newer non-enhanced techniques include quiescent-inflow single-shot (QISS) MRA, fresh blood imaging, and flow-sensitive dephasing, such as described in co-pending U.S. application Ser. No. 12/574,856, which is incorporated herein by reference in its entirety. QISS MRA has been shown to be a fast, accurate method for non-contrast MRA. The primary drawbacks are the need to synchronize the data acquisition to the electrocardiogram (ECG) and artifacts when severe arrhythmias are present.
In fact, most nonenhanced MRA techniques utilize cardiac gating. Notably, standard time-of-flight MRA does not utilize cardiac gating, but is, generally and unfortunately, not clinically useful for imaging outside of the head and neck. The need to apply ECG leads to the patient's chest is inconvenient and increases setup time for the MR examination. Moreover, ECG gating often fails when imaging is performed at high field strengths, such as 3 Tesla fields, due to interference from the magnetohydrodynamic effect. Moreover, some patients have a low QRS voltage or have irregular heart rhythms, which make detection of the ECG signal unreliable. Magnetic or RF interference from the scan process itself can also cause ECG gating to fail. In rare instances, the use of ECG leads has caused skin burns.
Thus, there remains a need to provide a method for non-contrast enhanced magnetic resonance angiography that is insensitive to patient motion; consistently and accurately portrays vessel anatomy, even in patients with severe vascular disease; and is less time consuming than existing methods.